Conventionally, this type of MESFET, as shown in FIG. 1, has an active layer (n-type layer) 2 on a GaAs substrate 1 having a high resistance, and a pair of contact layers (n.sup.+ -type layers) 31, 32 are formed on the active layer 2. On one contact layer (n.sup.+ -type layer) 31, a source electrode 6 is provided and, on the other contact layer (n.sup.+ -type layer) 32, a drain electrode 7 is provided, and a gate electrode 5 is provided on the active layer (n-type layer) 2 between the pair of contact layers (n.sup.+ -type layers) 31, 32 to form a recessed structure.
That is, the foregoing MESFET, in order to reduce the parasitic resistance, adopts a recessed structure in which the thickness of the active layer (n-type layer) of the source and drain areas is made greater than that of the active layer (n-type layer) below the gate electrode while the contact layer (n.sup.+ -type layer) is formed on the active layer of the source and drain areas.
On the other hand, in order to improve the gate-drain reverse breakdown voltage BV.sub.GD of the MESFET, there is a case in which a semiconductor layer having a reduced impurity density is formed at a portion nearer to the gate electrode between the gate and the drain to alleviate the concentration of the electric field around the gate edge (See Japanese Patent Appln. Laid-Open No. 64-61067).
FIG. 2 illustrates this type structure. The MESFET has the active layer (n-type layer) 2 on the high resistance GaAs substrate 1, and the source electrode 6 and the drain electrode 7 are disposed on the active layer 2, on which the gate electrode 5 is disposed between the source electrode 6 and the drain electrode 7. A semiconductor layer (n.sup.- -type layer) 4 having an impurity density lower than that of the active layer (n-type layer) 2 is formed at an area on the active layer 2 below an edge portion of the gate electrode 5.
However, in the foregoing conventional recessed type MESFET shown in FIG. 1, when the drain voltage is very high, the concentration of the electric field occurs in the active layer at the surface side near the drain side recess edge rather than the substrate side. That is, although, within the recess, the potential lies on the high level supported by the gate potential and the substrate potential, it is abruptly lowered in the neighborhood of the drain side recess edge toward the contact layer (n.sup.+ -type layer), and therein the intensity of the electric field becomes great.
Further, since the potentials of the drain electrode and the drain side contact layer (n.sup.+ -type layer) of the surface side become lower than the potential at the substrate side, channel stricture occurs and the current is concentrated toward the neighborhood of the surface, i.e. the drain side recess edge. That is, the concentration of the electric field and the current occurs in the neighborhood of the drain side recess edge and, in this area, generation of the electron-hole pairs caused by collision ionization becomes remarkable.
Although part of the electrons and holes generated flows as the current, part of them is spent for recombination of the electron-holes. As a result, if the recombination occurs at non-luminescence center, then the energy thereof is given to the crystal lattice of the semiconductor, which causes dislocation or proliferation of the defects in the crystal with the result that degradation of the characteristics of FET (reduction of the drain current) occurs.
Further, in the foregoing conventional structure shown in FIG. 2 in which the impurity density of a part of the semiconductor layer in the neighborhood of the gate electrode is reduced, although the electric field concentration in the neighborhood of the gate electrode, which occurs when the gate voltage is low, is alleviated, when the drain voltage is high, it does not lend itself to the electric field alleviation at the drain side recess edge.
As described above, in the conventional recessed type semiconductor device, when the drain voltage is very high, concentration of the electric field and the current at the recess edge cannot be alleviated, and the degradation of the characteristics caused by the collision ionization cannot be prevented.